Distribution
The bivalves are a highly successful class of invertebrates found in aquatic habitats throughout the world. Most are infaunal and live buried in sediment on the seabed, or in the sediment in freshwater habitats. A large number of bivalve species are found in the intertidal and sublittoral zones of the oceans. A sandy sea beach may superficially appear to be devoid of life, but there is often a very large number of bivalves and other invertebrates living beneath the surface of the sand. On a large beach in South Wales, careful sampling produced an estimate of 1.44 million cockles (Cerastoderma edule) per acre of beach.
Bivalves inhabit the tropics as well as temperate and boreal waters. A number of species can survive and even flourish in extreme conditions. They are abundant in the Arctic, about 140 species being known from that zone. The Antarctic scallop, Adamussium colbecki, lives under the sea ice at the other end of the globe, where the sub-zero temperatures mean that growth rates are very slow. The giant mussel, Bathymodiolus thermophilus, and the giant white clam, Calyptogena magnifica, both live clustered around hydrothermal vents at abyssal depths in the Pacific Ocean. They have chemosymbiotic bacteria in their gills that oxidise hydrogen sulphide, and the molluscs absorb nutrients synthesized by these bacteria. The saddle oyster, Enigmonia aenigmatica, is a marine species that could be considered amphibious. It lives above the high tide mark in the tropical Indo-Pacific on the underside of mangrove leaves, on mangrove branches and on sea walls in the splash zone.
Some freshwater bivalves have very restricted ranges. For example, the Ouachita creekshell mussel, Villosa arkansasensis, is known only from the streams of the Ouachita Mountains in Arkansas and Oklahoma, and like several other freshwater mussel species from the southeastern USA, it is in danger of extinction. In contrast, a few species of freshwater bivalves, including the golden mussel (Limnoperna fortunei), are dramatically increasing their ranges. The golden mussel has spread from south-east Asia to Argentina, where it has become an invasive species. Another well-travelled freshwater bivalve, the zebra mussel (Dreissena polymorpha) originated in southeastern Russia, and has been accidentally introduced to inland waterways in North America and Europe, where the species damages water installations and disrupts local ecosystems.
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